Is a person considered an infidel if he does not forbid evil despite being able to, after his female friend said something forbidden in jest and mockery?

Your leaving the gathering is not considered disbelief (kufr). It is your duty to forbid evil if it occurs in front of you. If you are unable to forbid it, then leave that gathering. If you did not think about it, despite your ability to do so, you would be sinful, and repentance and seeking forgiveness would be obligatory upon you. This is not disbelief. One should be wary of obsessive whispers (waswasah) and indulging in them.